A device of the aforementioned kind is disclosed in U.S. Pat. No. 5,821,811 (Allgon AB). In the known device, the amplifier unit forms a closed loop together with the bypass segment. In case the isolation across the bypass segment in active mode is lower than the amplifier gain, there is a risk of instability due to feedback through the bypass segment.
In order to secure sufficient stability, the isolation in the bypass segment should be at least 10 dB higher than the gain of the amplifier unit in active mode. When using the previously known device, which is provided with pin diodes as switching elements, it is necessary to use more than two switching elements, normally four switching elements to achieve an isolation of 30 dB. However, such an arrangement also leads to a relatively high insertion loss in the bypass mode of the device, namely in the order of 2 dB. This level is a drawback, since it adversely effects the operation of the device in the bypass mode.
A related problem of an amplifier/bypass device of the aforementioned kind is the difficulty in ensuring a proper operation of the device in active mode. Thus, it is desired to maintain a low noise factor and to avoid input/output reflections in the amplifier unit.
Such noise will appear even if two identical low noise amplifiers (LNA) are inserted in a balanced configuration between an input hybrid coupler and an output hybrid coupler, as is previously known per se from U.S. Pat. No. 4,825,177 (General Instrument Corporation). This is due to the fact that the bypass segment will alter the balance of the amplifier unit.